The use of rotating polygon scanning mirrors and laser printers to provide a beam sweep or scan of the image of a modulated light source across a photosensitive medium, such as a rotating drum, is well known. In addition, there have also been efforts to use a much less expensive flat mirror with a single reflective surface such as a mirror oscillating in resonance to provide the scanning beam.
For example, the use of a single flat mirror with a pair of torsional hinges has been used for providing the horizontal scan or sweep of the laser beam. The pair of torsional hinges provide the oscillating beam sweep, which is typically a resonant beam sweep. In addition, other devices having torsional hinges that oscillate at the resonant frequency of a device have been contemplated.
Consequently, it will be appreciated that the high frequency resonating device, such as a scanning mirror, is a key component to the success of products such as laser printers. Further, since many of the applications for such devices, including mirror projection displays, are battery powered, and therefore, all of the components including the scanning mirror or resonant movement of the device must be energy efficient. As an example, Texas Instruments presently manufactures mirror MEMS devices fabricated out of a single piece of material (such as silion, for example) typically having thickness of about 115 to 125 microns using semiconductor manufacturing processes. The layout of such a mirror consists of a mirror having dimensions on the order of a few millimeters pivotally supported by torsional hinges. The torsional hinges extend from the mirror member of the device to a support frame or alternately, the hinges may extend from the mirror device to a pair of hinge anchors. This Texas Instruments manufactured mirror is particularly suitable for use with laser printers on projection device displays. The reflective surface of the mirror may have any suitable perimeter shape such as oval, elongated ellipse, rectangular, square or other.
U.S. Patent Application No. 2004/0027449, filed Mar. 10, 2003 and entitled “Laser Print Apparatus Using a Pivoting Scanning Mirror” describes several techniques for creating the pivotal resonance of the mirror device or any other resonant member about torsional hinges. Thus, by designing the hinges to resonant at a selected frequency, an engine can be produced that, in the case of a mirror, provides a scanning beam sweep with only a small amount of energy required to maintain resonance.
However, as will be appreciated by one skilled in the art, the resonant frequency of a pivotally oscillating device about torsional hinges will vary as a function of the stress loading along the axis of the hinges. These stresses build up as a result of residual stress on the hinge from the assembly process as well as changes in the environmental conditions, such as for example, changes in the temperature of the package device. For example, the coefficient of thermal expansion of silicon is different than that of most packaging materials such as stainless steel or plastic. As the temperature varies, these materials expand and contract at different rates. A MEMS type pivotally oscillating device, which is made of Si and attached to the package is constrained in the hinge direction and will experience stress in the hinges as the temperature changes. This in turn will lead to drift in the resonant frequency of the pivotal oscillations.
Since applications that use a pattern of light beam scans, such as laser printing and projecting imaging, require a stable precise drive to provide the single frequency of the scan velocity, the changes in the resonant frequency and scan velocity of a pivotally oscillating device, such as the mirror, due to temperature variations can restrict or even preclude the use of the device in laser printers and scan displays. Further, if the stress loading of the torsional hinges is increased above the maximum acceptable level for a given rotational angle, the reliability and operational life of the device or mirror can be unacceptably reduced. For example, excessive compressing stress loading that can occur at a low temperature on a device with a CTE (coefficient of thermal expansion) mismatch can lead to buckling of the hinge along with dramatic shift of the resonant frequency or even catastrophic failure.
Various techniques are now used to reduce stress on the torsional hinge devices or mirrors due to temperature and other causes. Unfortunately, these techniques add cost to the overall device or mirror and support structure making up, for example, a laser drive package.
Therefore, an inexpensive torsional hinge device, such as a mirror, having a resonant frequency that is substantially indifferent to the effects of significantly different CTE's of a support structure and mirror combination and other sources of stress transmitted through the torsional hinges, would be advantageous.